Background. Inherited mutations of the breast cancer susceptibility gene-1 (BRCA1) confer a high risk for specific tumor types, including breast and ovarian cancers. BRCA1 plays roles in the regulation of DNA repair, cell cycle progression, apoptosis, and gene transcription. But these generic actions do not explain why BRCA1 mutations lead to specific tumor types, such as breast cancer. We have been studying a mammary tissue-specific action of BRCA1 [ie., its ability to repress estrogen receptor (ER-alpha signaling) that may, in part, explain why BRCA1 mutations are associated with estrogen (E2)-dependent tumor types (eg., breast, endometrial, and cervical cancers). Preliminary Studies. During the initial funding period, we have completed and gone beyond the original proposed objectives in understanding the mechanisms by which the BRCA1 regulates estrogen action. In new preliminary studies, we mapped the interacting sites between the BRCA1 and ER-alpha proteins at a high resolution. And we adduced evidence for two distinct mechanisms by which the ER-alpha regulatory function of BRCA1 can be inactivated: 1) through oncogenic signaling pathways (eg., over-expression of cyclin D1, c-Myc, and the c-Akt kinase); and 2) via a tumor-associated mutation of an acetylation site within the hinge region of ER-alpha. Finally, we generated preliminary evidence that exogenous E2 induces mammary hyperplasia and preneoplasia in Brca1-deficient mice. Hypothesis. BRCA1 prevents breast cancer, in part, by inhibition of ER-alpha activity and, in part, by estrogen (E2)-independent actions (ie., by its function as a "caretaker" gene, to prevent genomic instability). The E2-dependent and E2-independent functions of BRCA1 are distinct and can be structurally dissociated. Finally, the ability of BRCA1 to repress ER-alpha can be functionally inactivated by oncoproteins and growth factor signaling pathways. Research Aims. The major goals of this project are: SA1. To precisely identify the molecular determinants of the BRCA1:ER-alpha interaction; SA2. To determine the mechanisms by which oncogene signaling rescues BRCA1 inhibition of ER-alpha. SA3. To determine the role of acetylation in regulating ER-alpha resistance to inhibition by BRCA1; and SA4. To examine the interaction between estrogen and Brca1 in mammary tumorigenesis in vivo. Significance. These studies address a major mechanism by which BRCA1 inactivation leads to breast cancer: namely, deregulation of ER-alpha signaling. They are relevant to the pathogenesis of both hereditary and sporadic breast cancers. Thus, sporadic breast cancers frequently exhibit absent or reduced expression of BRCA1; and we found that knockdown of endogenous BRCA1 by RNA interference causes ligand-independent activation of ER-alpha. In addition to their implications for breast cancer pathogenesis, these studies are of practical importance, since they may suggest novel approaches for breast cancer prevention and treatment.